And then he tried to make nice with Russia, Syria’s ally, in the very next tweet.

President Donald Trump just tweeted a warning that an attack on Syria is imminent — breaking his own policy of not signaling upcoming military moves.

The tweet comes as Trump weighs how to punish Damascus after President Bashar al-Assad’s forces carried out an April 7 chemical attack on civilians, killing at least 40 people. Trump promised a big response and has consulted with his national security team...